Hydraulic actuation systems that use hydraulic power to facilitate mechanical motion (e.g. linear, rotary or oscillatory motion) have many uses across a range of technologies. Hydraulic actuators typically comprise a hollow tube along which a piston can slide and can be single-acting or double-acting. In a double-acting actuator, hydraulic fluid pressure is applied from a chamber on each side of a piston, and the pressure differential between the two chambers moves the piston one way or the other. The movement of the piston provides an actuation force.
A common use for double-acting hydraulic actuators is within propeller pitch control systems, such as pitch change actuators for variable pitch propellers. Variable pitch propellers are employed on many different types of vehicles, such as aircraft. Typically, propeller blades are mounted to a rotary hub for pivotable movement about their longitudinal axis to permit pitch adjustment. The pitch adjustment is controlled by a double-acting hydraulic pitch change actuator housed within the rotating hub assembly. On one side of the piston is an “increase pitch pressure chamber” and on the other side a “decrease pitch pressure chamber”, with the differential pressure between the two moving the piston so as to cause the pitch angle to increase or to decrease. The pitch change actuator is operated by a flow metering valve such as an electrohydraulic servo valve or direct drive servo valve, for selectively pressuring the pitch change actuator piston to effectuate a desired change in pitch of the propeller blades, which in turn is controlled by a closed-loop control system.
Pitch change actuators are well known in the prior art, for example in U.S. Pat. No. 8,439,640 B2.
In the closed-loop control systems of the prior art, the engine control system provides the inputs to a position loop, which controls the position of the propeller pitch actuator (and thereby the pitch of the blades). In order to minimise possible interactions with other powerplant systems, the position loop has a specific bandwidth and operates at a given frequency (i.e. the frequencies encompassed by the position loop bandwidth are distinct from the frequencies of other systems in order to allow decoupling from the engine power and rotational speed control loops for instance).
However, the present inventor has observed position loop instabilities induced by coupling phenomenon with other control loops embedded in the powerplant and running in parallel with the propeller position loop. Identified possible interactions are with:
the propeller and engine rotational speed regulation
the engine power regulation
the engine torque control
propeller synchrophasing loops
any controls embedded within engine turbomachinery
Aircraft control systems
These interactions often occur when the various loops operate at similar frequencies. Although the control loops will be designed to operate at different frequencies that should not interact, for many reasons the operating frequency of any of the control loops can shift over time (hardware ageing, manufacturing tolerances, change in aircraft & engine operating conditions, maintenance operations etc.), causing loops to then operate at similar frequencies and interact.
The present disclosure seeks to address the above described issues.
The thesis “Contribution a la modelisation et la commande des systemes electrohydrauliques”, Tafraouti M, Universite Henri Poincare, 17 Nov. 2006 discusses the theory behind control loops used in electrohydraulic systems. At chapter 2, section 2.5.4.2, use of feedback of the pressure differential or acceleration to improve the damping of the system and increasing the frequency range of the bandwidth is discussed. A preliminary theoretical analysis is presented of the potential benefits of adding an inner load loop to the classical position loop of a hydraulic system.
U.S. Pat. No. 7,104,053 describes a control method for controlling the operation of an actuation system comprising first and second actuator arrangements arranged to drive a common element, for example driving a flight control surface of an aircraft wing. The aim is to equalise the force applied by the two actuators, to avoid stress and system inefficiency. The method uses a single pressure sensor to measure the differential pressure between the two chambers of each actuator from which the load applied to the control surface by the actuator piston can be calculated. The demand signals applied to each actuator are adjusted to compensate for any difference in the actuator loads.
U.S. Pat. No. 8,474,752 similarly relates to flight control actuator force equalisation. Each of a plurality of actuators is provided with a force sensor that senses the pressure difference across the actuator piston, i.e. providing a delta pressure signal. The applied force per piston can thus be determined and the difference in forces between the actuators used to equalise the forces across the control surface.